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An Overview of Purple Non-Sulfur Bacteria

Updated: Feb 1, 2021

Purple non-sulfur (PNS) bacteria are flagellated, gram-negative proteobacteria. They have been a vital component of food webs for eons. Even though they are primitive in form, they can be quite sophisticated in their capacity to adapt to different environmental conditions. Key to this adaptability is their ability to switch between autotrophic and heterotrophic metabolic function [3,6].

photosynthetic bacteria

PNS bacteria are especially notable for their oftentimes intense pigmentation. Major pigments include carotenoids as well as bacteriochlorophyll a and b. While some species can indeed become quite purple, others may be anything from yellow to blood red. The ubiquitous soil/water bacterium Rhodopseudomonas palustris can vary considerably in color, though it characteristically takes on a rosey amber hue; its pigmentation has been described as light brown to peach-colored [10].


Ready for anything

R. palustris is said to be the one of the most metabolically versatile organisms that has ever existed. The species is naturally omnipresent, occurring in soils as well as freshwater and marine sediments. It can survive within an extreme range of temperature/pH/salinity and thrive in both aerobic and anaerobic conditions. It can respond rapidly to fluctuations of dissolved oxygen and nitrogen/organic carbon concentrations. Under extremely nutrient-poor conditions, it may fix nitrogen [4, 7]. While it can carry out photoautotrophy much like algae, it is additionally capable of "eating" organic matter and respiring under dark conditions (i.e. chemoheterotrophy) [8]. However, it is predominantly an anaerobic photoheterotroph that favors aromatic organic compounds and organic acids (e.g. fatty acids) as carbon sources.

Its carotenoids (max absorption ~475 nm) and bacteriochlorophylls (max absorption ~850 nm) can efficiently utilize the relatively low levels of light that penetrate into soils, sediments and deep waters (to ~150 m). It even alters the relative proportion of its pigments to adapt to specific environmental conditions. For example, when blue light is abundant, it mainly produces carotenoids (taking on a rusty orange color). During phytoplankton blooms or while growing under dense plant/macroalgae canopies, blue light can become scarce. Under these conditions, it mainly produces bacteriochlorophylls (taking on a purplish color) to scavenge red light.


How they "work"

Because PNS bacteria (1) displace pathogenic microbes and (2) strip dissolved wastes from the water column, they have for long been used successfully for water remediation in intensive aquaculture operations [15]. Horticulturists the world over have likewise begun to recognize the many ecological benefits of PNS bacteria; growers are reporting success using liquid PNS bacteria-based products in everything from hydroponic additives and soil drenches to foliar feeds [9]. PNS bacteria have even been shown (when added to drinking water) to boost productivity and quality in broiler chickens [1]! 

This versatile microbe promotes the health and productivity of any culture system. Its contributions are many:​

  • It reliably consumes organic matter in anaerobic zones without generating noxious byproducts such as hydrogen sulfide [11].

  • By sequestering organics, it significantly amends water quality and reduces odors while helping to extend the life of activated carbon products [15].

  • It assimilates NH4/NO2/NO3 and can carry our denitrification, significantly enhancing a grow system's overall  capacity for fast, natural nitrogen cycling [14].

  • It acts as a powerful probiotic, reducing the occurrence of disease in both plants and animals [15].

Even more, PNS bacteria are themselves regarded as a highly nutritious feedstuff for beneficial soil/sediment mesofauna as well as many zooplanktonic or filter-feeding organisms. Their nutritional benefits are considerable:

  • ​Their tiny cell size (0.6-0.9 x 1.2-2.0 µm) makes them easily ingestible for even the tiniest animals and the finest filter-feeders. 

  • Because their cell wall lacks cellulose, they are more easily digested than algae [13]. 

  • They have a high protein content and contain important B vitamins, lipids and biological cofactors [12, 13].

  • When photosynthetically active under bluish light, they are rich in healthful, color-enhancing pigments such as astaxanthin [12]. 

  • They promote zooplankton (e.g. copepod) productivity and may be used to nutritionally enrich live zooplanktonic feeds (e.g. brine shrimp)[2].

  • They have been identified as a potential superfood for finicky microplanktivores such as carnation corals [16].


The quintessential aquarium/hydroponic water inoculant?

Perhaps it shouldn't be so surprising that these primordial bacteria are so favorable for so many types of agriculture, being that they are at the base of virtually every earthly ecosystem [5]. Their usefulness in aquaculture and horticulture has been demonstrated repeatedly. While the investment in time and money to use them is modest, increases of growth/yield and improved health/appearance are (as numerous studies have shown) quite substantial.

Rhodopseudomonas palustris

Whether you're cultivating corals, pond lilies, tilapia or tomatoes, this indispensable organism should always be counted among your grow system's microbial community! PNS ProBio™ is produced under proprietary culture conditions that favor a high carotenoid content. 


Click HERE to learn more about PNS ProBio™.



[16] Delbeek, J. Charles and Julian Sprung. The Reef Aquarium: Science, Art, and Technology. Coconut Grove, FL: Ricordea Publishing, 2005.

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